Pumice

Pumice is a very light, porous volcanic rock that forms during explosive eruptions. During the eruption, volcanic gases
dissolved in the liquid portion of magma expand very rapidly to create a foam or froth; the liquid part of the froth then
quickly solidifies to glass around the gas bubbles. All types of magma (basalt,
andesite, dacite, and
rhyolite) can form pumice, but it is usually associated with acidic
magmas (e.g. Rhyolite). Pumice is mined as an additive to abrasive materials.

Pumice mine on Lipari, Eolian Islands, Italy. Pieces which fall into the sea float and can be found as far away as Stromboli.
Photo: J. Alean